Apple has confirmed that manually changing your date to May 1970 or earlier will result in a "continuous reboot"—which basically turns your iPhone into a paperweight. There is no "fix" to the issue at the moment, but Apple said help is on the way.

Manually changing the date to May 1970 or earlier can prevent your iOS device from turning on after a restart. An upcoming software update will prevent this issue from affecting iOS devices. If you have this issue, contact Apple Support.

One YouTuber offers his theory on why the phone goes haywire when the date is changed.